Neuropsychologists define "prosocial" people as those who prefer to share equally with others, and "individualists" as those who are primarily concerned with maximizing their own personal gain.  Masahiko Haruno, from Tamagawa University (Japan) and Christopher Frith, from University College London (United Kingdom) used functional MRI to scan the brains of 25 prosocial people and 14 individualists, as their preferences for a series of money distributions between themselves and a hypothetical other person was rated.  Those in the prosocial group preferred even splits while the individualists favored distributions where they got the most money. Interestingly, the researchers also found that the only brain region that differed in activity between the two groups was the amygdala:  unfair money distributions increased the activity in the amygdala in prosocial people but not in the individualists, with the greater the dislike over the distribution, the more prominent the brain activity in that region.  The team plans to identify how the difference in the activity of the amygdala arises, which may be partly genetic but is also expected to be influenced by a person's environment, he says, particularly the social interactions during childhood.